Mixed-ligand complexes of iron and hydroxy-1,10-phenanthrolines

The half-wave potentials of all of the possible tris-complexes formed between iron(II) and 1,10-phenanthroline (A), 4-hydroxy-1,10-phenanthroline (B), and 4,7-dihydroxy-1,10-phenanthroline (C) have been measured at a rotated platinum electrode. Values on the hydrogen scale are 1.06 V for A(3)Fe(II), 0.78 for A(2)B, 0.58 for A(2)C, 0.60 for AB(2), 0.39 for B(3), 0.39 for ABC, 0.21 for AC(2), 0.22 for B(2)C, 0.06 for BC(2), and -0.10 for C(3) at pH 11.0, 25 degrees , and ionic strength 0.2. Half-wave potentials of the parent binary complexes are constant from pH 9 to 13 and are equal to the conditional reduction potentials. The mixed complexes are stable from pH 10 to 12, and form a redox indicator system with a continuous range of accessible potentials from -0.10 to +1.06 V vs. NHE.